The present invention relates to electronic impact printers and more particularly to methods and means for protecting the hammer drive circuits used in effecting printing.
Printers exist today wherein a full line of input data characters are stored in memory, then processed sequentially before a plurality of such stored input data characters are printed along a line on a record medium. Printing takes place by the operation of hammers which cause type characters to impact the record medium. The number of hammers involved may number well over 100. Generally speaking, there is associated with each hammer a solenoid which is selectively energized at the appropriate time to cause hammer actuation to take place. For further details of the type of printer arrangement involved, reference may be made to U.S. Pat. No. 3,803,558 issued Apr. 9, 1974 to Clifford M. Jones et al entitled "Print Selection System". Reference may also be made to U.S. Pat. No. 3,605,610 issued on Sept. 20, 1971 to Earle B. McDowell et al entitled "Type Member Position Sensing System in a High Speed Printer". This latter patent describes the details of a hammer drive circuitry for use with a belt printer. A belt printer comprises a continuously moving character belt that carries the type faces for each character to be printed. The number of type faces carried on the belt depends upon the number of characters or symbols the apparatus is to be capable of printing. A plurality of hammers are arranged in a row across the face of a record medium such as paper, the position of each hammer establishing a column in which a character may be printed. An inking ribbon is positioned in front of the record medium and the path of the character belt is located behind the inking ribbon and in front of the hammers. Means are provided for discretely indicating the control circuitry where each character appears relative to the record medium. When this is known, circuitry is provided for energizing the hammers at an appropriate position to imprint the appropriate characters in any desired position. A common type of hammer firing circuit employs SCR's in series with respective hammer solenoids. At an appropriate time the SCR is gated on, thereby energizing the associated solenoid and causing the respective hammer to actuate the type character on the belt located at the column location associated with the particular hammer. Where hammers number in the order of 100 or more, it is apparent that a substantial amount of power is involved to enable simultaneous energization of a plurality of solenoids and hence simultaneous printing at various column locations on the record medium. The nature of the printing is such that extremely high currents must be delivered to the solenoids to cause swift hammer actuation as well as to drive the hammer with sufficient force to cause impact printing on the record medium including multiple copies thereof. The solenoid coils can withstand this high power application intermittently but not continuously. The nature of line printing or partial line printing is such that printing and hence solenoid operation does not occur continuously. However, difficulties arise with respect to the power application circuits associated with each solenoid, particularly where silicon controlled rectifiers (SCR's) are employed. Some SCR's may fail during operation by being unable to withstand a forward voltage or because of short circuiting in the SCR itself. In either case, control of SCR operation by a gate control signal is lost, giving the effect of a continuously closed switch resulting in overloading of the hammer solenoid.
Efforts to protect against SCR malfunction and prevent solenoid burn-up heretofore have been relatively unsuccessful or unreliable particularly at higher printing rates. Sometimes, monitoring the average hammer drive current at low printing rates is possible when the average current attributable to an SCR failure is greater than the total average hammer drive current associated with relatively low printing rates. However, when the printing rates result in an average current being supplied to the hammer solenoids which is greater than that due to the failure of one SCR circuit, this approach is impractical.
Accordingly, one object of this invention is to provide an improved apparatus for detecting SCR failure in a hammer drive circuit for a high speed line printer.
Another object of this invention is to provide an improved method and apparatus for protecting hammer drive solenoids against malfunctions in the switching circuits employed for applying power to the solenoids.
Another object of this invention is to provide an improved method and apparatus for halting printing whenever an SCR failure occurs in the hammer drive circuits.
In accordance with one embodiment, the invention is employed in a line printer which employs successive time periods for effecting printing. During one such period or phase, input character data is compared with the location of moving type fingers (carried on an endless belt moving past the various column locations on a record medium) and the column location at which the various input characters are to be printed to produce a hammer firing signal. The hammer fire signals developed are applied to respective hammer firing SCR's to precondition these SCR's to be operated during the following drive phase. During the drive phase, drive voltage is applied to all hammer drive SCR cicuits simultaneously and those which have been conducting previously cause the energization of the respective hammer solenoid. This, in turn, results in the actuation of a type finger and impact on the record medium to produce a recorded character. During the third or commutation phase, the SCR drive voltage is removed and a negative voltage applied sufficient to cut off all SCR's and reset them. Thus, any desired subsequent firing depends upon further comparisons of input character data, type finger location and column information. To protect the hammer drive solenoids against overload, a separate, single pilot SCR is provided for causing the printer to be shut down in the event no firing pulses are being developed when power is applied to the drive bus for energizing the hammer solenoids. However, if with the establishment of a drive voltage on the drive bus feeding the hammer drive SCR's there is detected a hammer firing signal for any one of the respective hammers, the printer is enabled to continue printing.